Hetero-Diels-Alder reactions of 1-oxa-3-butadienes with dienophiles is a powerful method for the construction of substituted dihydropyrans. The effect of Lewis acid catalysis in accelerating all carbon Diels- Alder reactions has been well advanced, however in hetero-Diels-Alder reactions, the prospects for catalysis are only beginning to be explored. This proposal concentrates on Lewis acid catalyzed hetero- Diels-Alder reactions using 2-exo-methylene-3-oxo-tetrahydropyrans as heterodienes. The cycloadducts which result represent fused bicyclic dihydropyran derivatives that are not otherwise accessible using current Diels-Alder methodology and would be valuable intermediates in the synthesis of biologically important natural products. It is proposed that, based on analogous literature precedent, the cycloadditions in question can be promoted by Lewis acid complexation on either the heterodiene component or the dienophile component. The design is such that the cycloadditions can be made to follow either normal or inverse electron demand, and can be dictated and controlled by the catalyst system employed. The strategy and rational of the appropriate heterodiene substrates, as well as the cycloadditions necessary to show their synthetic utility, will be presented.